Method and apparatus for shielding the effluent from plasma spray gun assemblies

ABSTRACT

Method and apparatus for plasma flame-spraying coating material onto a substrate by means of passing a plasma-forming gas through a nozzle electrode, passing an arc-forming current between said nozzle electrode and a rear electrode to form a plasma effluent, introducing spray coating material into the plasma effluent, passing the plasma effluent axially through a wall shroud extending from the exit of said nozzle electrode and forming a hot gas shroud for the plasma effluent at least within the wall shroud. .Iadd.

This Application is a Reissue Application of Pat. No. 4,121,082, issuedOct. 17, 1978. .Iaddend.

BACKGROUND OF THE INVENTION

This invention relates to the application of coatings onto substrates byplasma spray techniques, and more particularly, to method and apparatusfor shielding the effluent from plasma spray gun assemblies fromcontamination by the surrounding environment.

Plasma spray gun assemblies are known which use an electric arc toexcite a gas, thereby producing a thermal plasma or very hightemperature. Spray or powdered materials are introduced into the thermalplasma, melted and projected onto a substrate or base to form coatings.Such powdered materials may include metals, metal alloys, ceramics suchas metal oxides, and carbides or the like, for example.

Heretofore, difficulties were experienced due to contamination of theeffluent from the nozzle of the spray gun, such as air entrapment, forexample, that resulted in significant oxidation of the coatingmaterials. The spraying conditions, particularly heat and velocity, wereoften adjusted to a compromise to heat the powder just enough to meltit. Attempts have been made to overcome this problem, but they have beenonly moderately successful. One such attempt involved completelyenclosing the apparatus in a chamber, but this was expensive and alsovery cumbersome. In other installations, efforts were made to use a gasshroud to solve the problem. For example, the Jackson U.S. Pat. No.3,470,347 shows the use of a coaxial annular stream of unheated gas.However, this required a relatively large flow of gas, such as argon,which is expensive. In addition, there was a tendancy with such priorart devices to build up a coating on the shrouding device. Other relatedpatents in this art include Anderson et al, U.S. Pat. No. 2,951,143;Yoshiaki Arata et al, U.S. Pat. No. 3,082,314; and Unger et al, U.S.Pat. No. 3,313,909, for example.

SUMMARY OF THE INVENTION

The basic and general object of the present invention is the provisionof a new and improved method and apparatus, which overcomes or at leastmitigates some of the problems of the prior art.

A more specific object is the provision of method and apparatus whichprovides improvements in one or more of the following aspects: higherdeposition efficiency; reduced oxygen content in the effluent formetallic materials; reduced unmelted particle inclusions; increased feedrates; and improved quality of the coating.

To the accomplishment of the foregoing objectives, and additionalobjectives and advantages, which will become apparent as thisdescription proceeds, the invention contemplates, in one form thereof,the provision of a new and improved plasma spray gun assembly forcoating substrates which includes, in combination, a nozzle electrodehaving a nozzle passage therethrough, a rear electrode, and means forpassing plasma-forming gas through the nozzle electrode. In addition,the assembly includes means for passing an arc-forming current betweenthe electrodes to form a plasma effluent, and means for introducingcoating material into the plasma effluent. Further, the assemblyaccording to the invention, includes a wall shroud for the plasmaeffluent extending from the exit of the nozzle electrode, and means forforming a hot gas shroud for the plasma effluent within the wall shroudand in some instances extending beyond the wall shroud.

In one preferred form of the invention, the hot gas shroud is directedat an angle of between about 160° and about 180° with respect to theaxis of the plasma effluent, and more preferably, the hot gas shroud isdirected at an angle of about 180° with respect to the axis of theplasma effluent.

According to an aspect of the invention, the wall shroud is cylindricaland means are provided for water cooling this shroud.

According to another aspect of the invention, the means for forming ahot gas shroud for the plasma effluent at least within the wall shroudcomprises means for preheating the gas for said hot gas shroud, which invarious forms include an electric gas preheater, a second plasma flamegun assembly serving as a gas preheater, or an internal passageway inthe wall shroud which serves as a gas preheater.

In another form of the invention, an annular manifold is mountedadjacent the outer end of the wall shroud, which has jet orifice meansfor providing an annular curtain effect around the plasma flame as itleaves the wall shroud and passes towards the target substrate.

The invention, in another form thereof, is directed to a process forplasma flame-spraying coating material onto a substrate, which includesthe steps of: passing a plasma-forming gas through a nozzle electrode,and passing an arc-forming current between the nozzle electrode and arear electrode to form a plasma effluent. The process further includesthe steps of introducing coating material into the plasma effluent,passing the plasma effluent through a wall shroud extending from theexit of the nozzle electrode, and forming a hot gas shroud for theplasma effluent at least within the wall shroud. It will be appreciatedthat the coating material may be in any form suitable for plasmaspraying such as, for example, a solid wire or rod. However, powder ispreferable. The powder may be free flowing or in a binder such as aplastic bonded wire or the like, for example. The spray materialintroduced into the plasma effluent may be introduced at any convenientlocation, including one upstream of the arc. However, it is generallyintroduced at a point downstream of the arc, and preferably, downstreamadjacent the nozzle exit. Further, several points of introduction may beutilized simultaneously.

According to the invention, the hot gas shroud is preferably directed atan angle of about 180° with respect to the axis of the plasma effluent.Preferably, the gas for forming the hot gas shroud is preheated to atemperature above about 300° C. and, more preferably, the gas ispreheated to a temperature of between about 500° C. and about 1000° C.In a preferred form of the invention, the gas is a reducing gas or aninert gas selected from the group consisting of nitrogen, argon andhelium, and in some installations, a small amount of combustion gas isadded. Preferably, the flow rate of the hot gas is above about 500 cubicfeet per hour and, more preferably, the flow rate is between about 1000cubic feet per hour and about 2000 cubic feet per hour at a temperatureof about 500° C.

As another aspect of the invention, the process includes the step offorming an annular fluid curtain around the plasma effluent as it leavesthe wall shroud and passes towards the target substrate.

There has thus been outlined rather broadly the more important featuresof the invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention which will be described more fullyhereinafter. Those skilled in the art will appreciate that theconception on which this disclosure is based may readily be utilized asthe basis for the design of other methods and apparatus for carrying outthe several purposes of the invention. It is important, therefore, thatthis disclosure be regarded as including such equivalent methods andapparatus as do not depart from the spirit and scope of the invention.

Several embodiments of the invention have been chosen for purposes ofillustration and description, and are shown in the accompanyingdrawings, forming a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a medial sectional view of a plasma flame spray gun assemblyconstructed in accordance with the concepts of the present invention;

FIG. 2 is a sectional view taken along the line indicated at 2--2 inFIG. 1;

FIG. 3 is a fragmentary, medial sectional view showing the outletportion of the plasma flame spray gun, according to still anotherembodiment of the invention;

FIG. 4 is a medial sectional view of a plasma flame spray gun assemblyaccording to another embodiment of the invention;

FIGS. 5 to .[.9.]. .Iadd.7 .Iaddend.are schematic drawings each showinga wall shroud and hot gas shroud arrangement according to otherembodiments of the invention; and

FIG. .[.10.]. .Iadd.8 .Iaddend.is a table showing comparative testresults of a plasma flame spray gun according to the invention withrespect to conventional guns.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment of the invention illustrated in FIG. 1, a plasma spraygun assembly, indicated generally at 10, for coating a substrate 11,includes a nozzle electrode 12 having a nozzle bore or passage 14therethrough, and a rear electrode 16 mounted on an electrode holder 18.Electrical cable connections 20 and 22 serve to connect the electrodesto a suitable electrical source. A plasma-forming gas such as nitrogen,argon, helium, hydrogen or the like, for example, is passed from asuitable pressure source through a connector 24 into the space 14 aroundthe tip of the electrode 16, through an annular passage formed by theelectrode tip and the tapered portion of the nozzle. The current iscaused to flow from the connector 20 through the electrode holder 18 tothe electrode 16 and from the tip of the electrode 16 in the form of anarc to the nozzle 12 and then to connector 22, to thereby form a veryhot plasma flame which extends out through the exit 26 of the nozzleelectrode 12. One or more secondary gases can be mixed with the primarygas, if desired.

Heat fusible powdered coating material, such as powdered metal, orceramics or the like, for example, is entrained in a carrier gas, which,for example, may be a gas such as nitrogen, helium, argon, or even air,received from a suitable source through a connection 28 provided for thepurpose. In the embodiment illustrated, the powdered material isinjected into the plasma flame adjacent the nozzle exit 26, as by meansof a nozzle 30. As a result in operation, the plasma effluent or flamewith the powdered material carried therewith passes in the directionindicated by arrow 32 at a very high velocity, the axis thereof beingindicated at 33.

According to the invention, an annularly-shaped wall shroud, indicatedat 34, is mounted on the nozzle 12 adjacent the nozzle exit 36 to form ashroud chamber 37. In the embodiment illustrated, the wall shroud 34 iscylindrical, having an inner step portion 38 and an outer step portion40.

Still referring to FIG. 1, an annular plenum chamber 44 is mounted atthe outer end of the wall shroud 34 for feeding a plurality of jetorifices 46 that are directed at an angle of between about 160° andabout 180° with respect to the axis 33 of the plasma effluent or flame.Preferably, the jet orifices are directed at an angle of about 180° withrespect to the axis 33 of the plasma effluent to form anannularly-shaped hot gas shroud within the chamber, adjacent the wallshroud, as indicated by arrows 48. The gas forming this hot gas shroudis flowing at a high velocity and is in a turbulent state.Alternatively, the jet orifices may be in the form of a continuousnarrow annular slit-like opening. The hot gas for the hot gas shroud isfed to the plenum chamber 44 through an inlet 50 from a heating device52. The gas is heated in the heating device to a temperature above about300° C., with the upper limit being 2000° C. or above, the actual upperlimit being determined by the materials employed. The preferabletemperature range is between about 500° C. and about 1000° C. Anysuitable type of inert or reducing gas may be employed such as,nitrogen, argon or helium, for example. In some installations, a smallquantity of combustion gas, less than 50%, may be added as a getteragent for oxygen in the environment. Suitable combustion gases includepropane or hydrogen, for example. The flow rate of the hot gas in thehot gas shroud is above about 500 cubic feet per hour and preferablyfrom about 1000 cubic feet per hour to about 2000 cubic feet per hour ata temperature of about 500° C. The flow rate of the gas is inverselydependent upon the temperature so that the higher the temperature of thegas, the lower the flow rate required.

The heating device 52 may be of any suitable type such as, for example,an electric heater. A plasma flame gun assembly similar to thatdescribed hereinbefore, but without the addition of the powdered coatingmaterial, is particularly desirable for use as a hot gas source.

Due to the high temperatures involved with plasma spray guns of thisnature, water cooling may be provided. In such an installation, theelectrical cable connections 20 and 22 are constructed so as to receivewater cooled electric cables through which cooling water is forced. Thiscooling water flows through the connection 22 and around the nozzle 12,and then outwardly through one side and then inwardly through the otherside of a water jacket 56 to cool the wall shroud 34. The cooling waterthereafter is directed through a passage 58 to cool the electrode 16before passing out of the system through the connection 20.

It will be appreciated that the hot gas shroud, as indicated by arrow48, within the wall shroud 34 is directed towards the exit flow of thearc plasma flame, as indicated by arrow 32. The combination of these twoflows, together with the high temperature of the gases satisfies the arcplasma jet's characteristic aspiration of the surrounding atmospherewithout the plasma jet being either quenched by a cold gas stream orentraining air, which otherwise has a propensity to produce anuncontrolled oxidizing reaction with the material being sprayed. Thecharacteristics of the gas supplied to the plenum chamber 44 arecontrolled. Depending on the particular material being sprayed, thesegases may be adjusted to provide either oxidizing, neutral or reducingatmosphere both within the chamber 37 and beyond the exit thereof. Thisenables the chemical composition of the spray coating to be controlledsuch as, for example, controlling the carbon content of carbides, ironor the like and, also, compounds such as barium titanate may be sprayedwithout the usual reduction of oxygen content. In general, the sprayingof metals requires a reducing atmosphere, whereas when sprayingceramics, it is desirable to provide an excess of oxygen.

In certain installations, an annular manifold 59, FIG. 3, is mounted onthe outer end of the gas burner 412. Cooling water or an inert gas suchas, for example, nitrogen or argon is supplied to this manifold throughan inlet 61, and annular jet orifice outer means 60 are provided on theside of the manifold towards the substrate 11 to provide an annularcurtain effect around the plasma flame, as indicated by arrow 62. Notonly does the jet spray serve to shield the spray stream, it also allowsthe spray cone to be controlled and furthermore serves to provide somecooling of the substrate. Similarly, the same manifold may be used withpropane to provide a secondary flame shroud around the spray stream andthereby further reduce the oxide content of the coating. In certaininstallations it is desirable to utilize carbon dioxide for thispurpose.

FIG. 4 shows another embodiment of the invention wherein the gas for thehot gas shroud is preheated by a regenerative process, in which theplasma effluent, itself, heats the wall shroud. The plasma effluent 64passes longitudinally along its axis 66 through an annular wall shroud68. The wall shroud has an inlet 70 for receiving the gas and aninternal passageway 72 of generally serpentine configuration leading toan annular plenum chamber 74 located towards the outer end of the wallshroud. The plenum chamber feeds a plurality of jet orifices 76 or othersuitable nozzle-like apertures to direct the flow of hot gas, asindicated by arrow 78, at an angle of between about 160° and about 180°,preferably about 180°, with respect to the axis 66 of the plasmaeffluent 64. In operation, the gas is heated as it flows through theinternal passageway 72 so that by the time it is discharged through thejet orifices 76, the temperature thereof is in the desired ranges, asset forth hereinbefore in connection with the embodiment of FIG. 1.

While the embodiments of FIGS. 1 and 4 are the presently preferredembodiments, other desirable embodiments of the invention areillustrated in FIGS. 5 to .[.9. FIG. 5 shows in schematic form anannular wall shroud 80 with plasma flame or effluent 82 passinglongitudinally therethrough along an axis indicated at 84. In thisembodiment, an annular hot gas shroud 86 is directed parallel to thedirection of flow of the plasma effluent.

In the embodiment of FIG. 6, the plasma effluent 82 passeslongitudinally along its axis 84 through an annular wall shroud 88, andan annular hot gas shroud 90 is directed at an angle having a componentextending parallel to the direction of flow of the plasma effluent..]..Iadd.7. .Iaddend.

Referring next to the embodiment of FIG. .[.7.]. .Iadd.5.Iaddend., theplasma effluent 82 passes longitudinally along its axis 84 through anannularly-shaped wall shroud 92, and a portion of the gas for formingthe hot gas shroud is introduced, as indicated at 94, at an angle ofabout 180° with respect to the axis 84 of the plasma effluent, and asecond portion of the gas for forming the hot gas shroud is introduced,as indicated at 96, at an angle having a component extending parallel tothe direction of flow of the plasma effluent.

In the embodiment of FIG. .[.8.]. .Iadd.6.Iaddend., the plasma effluent82 passes longitudinally along its axis 84 through an annular wallshroud 98, and an annular hot gas shroud 100 is directed at an anglehaving a component extending in a direction opposite to the direction offlow of said plasma effluent.

FIG. .[.9.]. .Iadd.7 .Iaddend.shows an embodiment of the inventionwherein the plasma effluent 82 passes longitudinally along the axis 84through an annular wall shroud 102. A portion of the gas for forming thehot gas shroud is introduced, as indicated at 104, at an angle of about180° with respect to the axis 84 of the plasma effluent and a secondportion of the gas for forming said hot gas shroud is introduced, asindicated at 106, at an angle having a component extending in adirection opposite to the direction of flow of the plasma effluent.

It will be appreciated that the characteristics of the hot gas as setforth in detail in connection with the embodiment of FIG. 1 areapplicable to the embodiments of FIGS. 4 to .[.9.]. .Iadd.7. .Iaddend.

Thus, it will be appreciated that the gas for forming the hot gas shroudmay be introduced at one or more inlets and each inlet may be disposedat any angle from about zero to about 180° .[., and may even be normalto the direction of flow of the plasma effluent.]..

In order to more fully illustrate the nature of the invention, FIG..[.10.]. .Iadd.8 .Iaddend.presents a table indicating the comparativetest results, spraying the same material, of a conventional plasma spraygun assembly without shrouding and a plasma spray gun assemblyconstructed according to the invention. The test results show a clearsuperiority of the spray gun assembly of the present invention.

It will thus be seen that the present invention does indeed provide anew and improved plasma spray gun assembly which is superior toconventional spray guns with respect to deposition efficiency, reducedoxide contents, reduced unmelted particle inclusions, as well as otheroperative characteristics.

Having thus described the invention with particular reference to thepreferred forms thereof, it will be obvious to those skilled in the artto which the invention pertains, after understanding the invention thatvarious changes and modifications may be made therein without departingfrom the spirit and scope of the invention, as defined by the claimsappended hereto.

What is claimed is:
 1. A plasma spray gun assembly for coatingsubstrates comprising, in combination:a nozzle electrode having a nozzlepassage therethrough; a rear electrode; means for passing plasma-forminggas through the nozzle electrode; means for passing an arc-formingcurrent between said electrodes to form a plasma effluent; means forintroducing spray coating material into the plasma effluent; a wallshroud for said plasma effluent extending from the exit of the nozzleelectrode; and means for forming a hot gas shroud for said plasmaeffluent at least within the wall shroud .Iadd.directed at an angle suchthat the gas has a component of flow extending in a direction oppositeto the direction of flow of the plasma effluent. .Iaddend.
 2. A plasmaspray gun assembly according to claim 1 wherein said spray coatingmaterial is in the form of a powder.
 3. A plasma spray gun assemblyaccording to claim 1 wherein said means for forming a hot gas shroud forsaid plasma effluent at least within the wall shroud comprises means fordirecting said hot gas shroud at an angle of between about 160° to about180° with respect to the axis of the plasma effluent.
 4. A plasma spraygun assembly according to claim 1 wherein said means for forming a hotgas shroud for said plasma effluent at least within the wall shroudcomprises means for directing said hot gas shroud at an angle of about180° with respect to the axis of the plasma effluent.
 5. A plasma spraygun assembly according to claim 4 wherein said means for forming a hotgas shroud for said plasma effluent at least within the wall shroudincludes an annular plenum chamber having jet orifice means directed atan angle of about 180° with respect to the axis of the plasma effluent.6. A plasma spray gun assembly according to claim 1 further comprisingmeans for water cooling said wall shroud.
 7. A plasma spray gun assemblyaccording to claim 1 wherein said wall shroud is of cylindricalconfiguration.
 8. A plasma spray gun assembly according to claim 1wherein said means for introducing spray coating material into theplasma effluent is disposed adjacent the exit of the electrode nozzle.9. A plasma spray gun assembly according to claim 1 wherein said meansfor forming a hot gas shroud for said plasma effluent at least withinthe wall shroud includes an electric heater for preheating the gas forsaid hot gas shroud.
 10. A plasma spray gun assembly according to claim1 wherein said means for forming a hot gas shroud for said plasmaeffluent at least within the wall shroud includes a second plasma flamegun assembly for preheating the gas for said hot gas shroud.
 11. Aplasma spray gun assembly according to claim 1 wherein said means forforming a hot gas shroud for said plasma effluent at least within thewall shroud includes an internal passageway of generally serpentineconfiguration in said wall shroud for preheating the gas for said hotgas shroud.
 12. A plasma spray gun assembly according to claim 1 whereinsaid means for forming a hot gas shroud for said plasma effluent atleast within the wall shroud includes means for preheating the gas forsaid hot gas shroud to a temperature of from about 500° C. to about1000° C.
 13. A plasma spray gun assembly according to claim 1 whereinsaid means for forming a hot gas shroud for said plasma effluent atleast within the wall shroud includes means for introducing hot gas at aflow rate of between about 1000 cubic feet per hour and about 2000 cubicfeet per hour at a temperature of about 500° C. to form said hot gasshroud.
 14. A plasma spray gun assembly according to claim 1 whereinsaid hot gas shroud is formed of an inert gas.
 15. A plasma spray gunassembly according to claim 14 wherein said inert gas is selected fromthe class consisting of nitrogen, argon and helium.
 16. A plasma spraygun assembly according to claim 15 wherein said hot gas shroud furthercomprises a combustible gas.
 17. A plasma spray gun assembly accordingto claim 1 further comprising means for forming an annular curtaineffect around the plasma effluent as it leaves the wall shroud andpasses towards the substrate.
 18. A plasma spray gun assembly accordingto claim 17 wherein said means for forming an annular curtain effectincludes an annular manifold and orifice means mounted adjacent theouter end of said wall shroud. .[.19. A plasma spray gun assemblyaccording to claim 1 wherein said means for forming a hot gas shroud forsaid plasma effluent at least within the wall shroud comprises means fordirecting said hot gas at an angle having a component extending parallelto the direction of flow of said plasma effluent..]. .[.20. A plasmaspray gun assembly according to claim 1 wherein said means for forming ahot gas shroud for said plasma effluent at least within the wall shroudcomprises means for directing said hot gas at an angle having acomponent extending in a direction opposite to the direction of flow ofsaid plasma effluent..].
 21. A plasma spray gun assembly according toclaim 5 further comprising second jet orifice means directed at an angleof from about zero degrees to about 180° with respect to the axis of theplasma effluent.
 22. A plasma spray gun assembly according to claim 5further comprising second jet orifice means directed at an angle havinga component extending parallel to the direction of flow of said plasmaeffluent.
 23. A plasma spray gun assembly according to claim 5 furthercomprising second jet orifice means directed at an angle having acomponent extending in a direction opposite to the direction of flow ofsaid plasma effluent.
 24. A plasma spray gun assembly according to claim1 wherein said wall shroud has a radially-inwardly directed lip portiondisposed towards the exit end thereof.
 25. A process for plasmaflame-spraying coating material onto a substrate, which comprises thesteps of:passing a plasma-forming gas through a nozzle electrode;passing an arc-forming current between said nozzle electrode and a rearelectrode to form a plasma effluent; introducing coating material intothe plasma effluent; passing the plasma effluent longitudinally througha wall shroud extending from the exit of said nozzle electrode; andforming a hot gas shroud for said plasma effluent at least within thewall shroud .Iadd.directed at an angle such that the gas has a componentof flow extending in a direction opposite to the direction of flow ofthe plasma effluent. .Iaddend.
 26. A process for plasma flame-sprayingcoating material onto a substrate according to claim 25 wherein saidcoating material is in a powder form.
 27. A process for plasmaflame-spraying coating material onto a substrate according to claim 25wherein said hot gas shroud is directed at an angle of between about160° to about 180° with respect to the axis of the plasma effluent. 28.A process for plasma flame-spraying coating material onto a substrateaccording to claim 27 wherein said hot gas shroud is directed at anangle of about 180° with respect to the axis of the plasma flame.
 29. Aprocess for plasma flame-spraying coating material onto a substrateaccording to claim 25 further comprising the step of passing coolingwater through said wall shroud.
 30. A process for plasma flame-sprayingcoating material onto a substrate according to claim 25 wherein saidcoating material is introduced into the plasma effluent adjacent theexit of the electrode nozzle.
 31. A process for plasma flame-sprayingcoating material onto a substrate according to claim 25 wherein saidstep of forming a hot gas shroud for said plasma effluent at leastwithin the wall shroud includes the step of passing the gas for formingsaid hot gas shroud through an electric preheater.
 32. A process forplasma flame-spraying coating material onto a substrate according toclaim 25 wherein said step of forming a hot gas shroud for said plasmaeffluent at least within the wall shroud includes the step of using asecond plasma flame gun assembly for preheating the gas for said hot gasshroud.
 33. A process for plasma flame-spraying coating material onto asubstrate according to claim 25 wherein said step of forming a hot gasshroud for said plasma effluent at least within the wall shroud includesthe step of passing the gas for said hot gas shroud through an internalpassageway of generally serpentine configuration in said wall shroud.34. A process for plasma flame-spraying coating material onto asubstrate according to claim 25 wherein said step of forming a hot gasshroud for said plasma effluent at least within the wall shroud includesthe step of preheating the gas for said gas shroud to a temperatureabove about 300° C.
 35. A process for plasma flame-spraying coatingmaterial onto a substrate according to claim 25 wherein said step offorming a hot gas shroud for said plasma effluent at least within thewall shroud includes the step of preheating the gas for said gas shroudto a temperature of between about 500° C. and about 1000° C.
 36. Aprocess for plasma flame-spraying coating material onto a substrateaccording to claim 25 wherein the gas for said hot gas shroud is areducing gas.
 37. A process for plasma flame-spraying coating materialonto a substrate according to claim 25 wherein the gas in said hot gasshroud is in a turbulent state.
 38. A process for plasma flame-sprayingcoating material onto a substrate according to claim 25 wherein the gasfor said hot gas shroud is an inert gas.
 39. A process for plasmaflame-spraying coating material onto a substrate according to claim 38wherein said inert gas is selected from the group consisting ofnitrogen, argon and helium.
 40. A process for plasma flame-sprayingcoating material onto a substrate according to claim 25 wherein the gasfor forming said hot gas shroud includes a combustible gas.
 41. Aprocess for plasma flame-spraying coating material onto a substrateaccording to claim 25 wherein the flow rate of said gas in said hot gasshroud is above about 500 cubic feet per hour.
 42. A process for plasmaflame-spraying coating material onto a substrate according to claim 41wherein the flow rate of the gas for forming said hot gas shroud isbetween about 1000 cubic feet per hour and about 2000 cubic feet perhour at a temperature of about 500° C.
 43. A process for plasmaflame-spraying coating material onto a substrate according to claim 25wherein said coating material is a fusible powdered metal.
 44. A processfor plasma flame-spraying coating material onto a substrate according toclaim 25 wherein said coating material is a ceramic material.
 45. Aprocess for plasma flame-spraying coating material onto a substrateaccording to claim 25 wherein said coating material is a carbide.
 46. Aprocess for plasma flame-spraying coating material onto a substrateaccording to claim 25 further comprising the step of forming a fluidannular curtain around the plasma effluent as it leaves the wall shroudpassing towards said substrate. .[.47. A process for plasmaflame-spraying coating material onto a substrate according to claim 25wherein said hot gas shroud is directed at an angle having a componentextending parallel to the direction of flow of said plasma effluent..]..[.48. A process for plasma flame-spraying coating material onto asubstrate according to claim 25 wherein said hot gas shroud is directedat an angle having a component extending in a direction opposite to thedirection of flow of said plasma effluent..].
 49. A process for plasmaflame-spraying coating material onto a substrate according to claim 25wherein a portion of the gas for forming said hot gas shroud isintroduced at an angle of about 180° with respect to the axis of theplasma effluent and a second portion of the gas for forming said hot gasshroud is introduced at an angle of from about zero degrees to about180° with respect to the axis of the plasma effluent.
 50. A process forplasma flame-spraying coating material onto a substrate according toclaim 25 wherein a portion of the gas for forming said hot gas shroud isintroduced at an angle of about 180° with respect to the axis of theplasma effluent and a second portion of the gas for forming said hot gasshroud is introduced at an angle having a component extending parallelto the direction of flow of said plasma effluent.
 51. A process forplasma flame-spraying coating material onto a substrate according toclaim 25 wherein a portion of the gas for forming said hot gas shroud isintroduced at an angle of about 180° with respect to the axis of theplasma effluent and a second portion of the gas for forming said hot gasshroud is introduced at an angle having a component extending in adirection opposite to the direction of flow of said plasma effluent.